Nobody knows for sure. Rapid eye movement, or “REM” sleep, the form of sleep associated with vivid dreaming, can last up to about 45 minutes but we really have no definitive method for timing dreams. Subjective estimates of dream length, however, are proportional to length of dream reports. This is consistent with the theory that dreams can last a long time rather than flash by in an instant as some early dream theorists conjectured.
However, having said all that, the unconscious mind is very clever and can transmit a great deal of information in condensed form in a dream, which may allow for dream content to be quite rich, even in short bursts. The normal sleep cycle consists of four stages, stage 4 being when we dream; we enter into stage 4 some four to five times per night, so it is quite possible that during each stage “dreamlets” can occur, lasting a few seconds to 20-30 minutes.